"我在高中裡必須勤力地讀好英文"

你咁用唔係好啱…

I must study English in the senior form assiduously.
(我一定要在高中裡勤力地讀英文)
Note: 我在英國讀大學，他們是用Assiduous作勤力的意思的

When I am a senior form student, I must to work hard on my English
(當我是一位高中學生時，我一定要努力學好英文)

I must study hard for English in senior highschool.